The aims of the proposed work are to test five major hypotheses concerning the development of behavior and social relationships in mammals. The overall strategy used will be to describe the emergence of individual behavioral phenotypes in a free-living mammal; natural variation in early experience andjuvenile behavior in three cohorts of juveniles will be compared to subsequent variability in adult behavior. The hypotheses to be tested are: 1) juvenile play behavior is adaptive; 2) birth order related size differences among juveniles determine spacing and interaction patterns among juveniles; 3) juvenile dominance relationships predict adult dominance relationships and adult reproductive success; 4) social bonds among adults are predicted by spacing patterns among jeveniles; 5) social bonds among adults are predicted by interaction patterns among juveniles. The species to be used as a mammalian model in this work is the pronghorn (Antilocapra americana). It is easily observable and has a simple behavioral repertoire composed of discrete, quantifiable acts. Preliminary investigation revealed that there is abundant variability in juvenile early experience and behavior. At the study site (the National Bison Range, Moiese, MT) the animals are habituated to vehicles (thus easily observalbe), are protected from grazing competition and hunting, and are permanently enclosed. Individuals marked as fawns can be followed throughout their lives. The long-term importance of the work is that it will show in which respects natural variation in juvenile early experience and behavior leads to important differences among adults. This is a part of the information necessary to understand how the developing mammalian brain adaptively responds to the social environment, modifying its own growth and ultimately producing a distinct adult behavioral phenotype.